"I didn't ask you to look at it," Franco told Letterman when the host showed the actor's famous semi-nude shot where he's got a hand over his privates. "It's what the people want. If you don't want to see it, don't follow me. Don't follow me on Instagram!"

Franco said that when he posts photos of himself "it gets a lot of attention," but he just does it for the fun of it and it's not something he puts "a ton of thought into." And attention he does get, as he currently has over 2 million followers on Instagram.

But, the 127 Hours star said that his attention-getting hijinks on the social site are not due to loneliness or anything of the sort. "I have people around me all the time," he said.

"It's just that as a performer, you get used to interactions with attention and interactions with audiences, so it feels like an audience that's both private and large at the same time."

Franco added that he has discovered how powerful the selfie can be and that "attention is power," as he noted in his New York Times essay. "If you are someone people are interested in, then the selfie provides something very powerful."